Abstract Objective To assess the association of body composition (BC), evaluated by bioimpedance analysis (BIA), with disease activity, physical function and mobility in patients with axSpA undergoing bDMARD treatment for one year. Methods Patients with AS (radiographic axSpA) were enrolled in an extension of the German Spondyloarthritis Inception Cohort (GESPIC). Patients were required to be candidates for bDMARD therapy at baseline presenting high disease activity despite previous treatment with nonsteroidal anti-inflammatory drugs. Outcomes (disease activity, function and mobility) and BC parameters were assessed at baseline and every 6 months thereafter. The association between BC parameters and outcomes over 1 year was analysed using longitudinal generalized estimating equations. Results Seventy-four patients with radiographic axSpA were included in current analysis with a mean age of 36.5 years, disease duration of 6.2 years and Axial Spondyloarthritis Disease Activity Score (ASDAS) of 3.4 at baseline. Fat mass value and fat mass index were positively associated with disease activity (ASDAS: ß = 0.012, 95% CI [−0.003, 0.027] and ß = 0.035, 95% CI [−0.010, 0.081], respectively) and functional disability (BASFI: ß = 0.037, 95% CI [0.004, 0.070] and ß = 0.125, 95% CI [0.031, 0.219], respectively). Visceral adipose tissue (VAT) was associated with reduced spine mobility (BASMI: ß = 0.20, 95% CI [0.07, 0.33]). Additionally, increase in VAT and fat mass parameters was linked to worse disease activity and functional disability in women, while they were strongly associated with reduced spinal mobility in men. Conclusion Higher levels of body fat and VAT were positively associated with increased disease activity, functional disability, and reduced spinal mobility in patients with radiographic axSpA treated with bDMARDs.